Its definitely not the size of your databases that is causing the problem, my music and movie DBs are larger and XL loads them with no problem. As for quirky characters I think the term that should be used is 'illegal characters'. Look for (') apostrophes, (#) pound and (&) single ampersands in your databases as they are illegal characters/values in XML field names and will cause the problems you are experiencing.
I.E.
This is bad a bad XML title -> Joe's_#4 concerto_for_banjo&flute
This would work -> Joes_number_4_concerto_for_banjo_and_flute
The easiest way to do this would be to open your music.xml database in notepad and just do a search one for each of the characters (', #, &) and if you find an entry with the offending characters - go to that mp3 file and edit the title to remove any illegal characters - then either re-import or edit the XML DB to match the new name.
I should probably add this info to my guide as XML data schemas aren't exactly second nature to most people.
For those of you looking for more detailed programming info on legal/illegal XML characters refer to this:
http://www.w3.org/TR/REC-xml
and look at at section 2.2.